During welding of alloy metals, the weld area should be out of contact with air or any other oxygen-bearing gas. By providing an inert gas in the weld region, contamination of a molten weld puddle from surrounding oxygen is prevented. Commonly, inert shielding gases are provided to a molten weld puddle via an annular passage-way surrounding the welding torch. Such an inert gas supply provides a limited region of inert gas beneath the torch. The prior art includes many teachings of weld shielding structures. In tube welding, an inert purging gas has been introduced into the tube. For stationary weld joints, particularly circumferential seams, the earliest method, which is still used today, involves sealing the ends of the tube and filling the tube with the inert gas. Such procedures are shown in U.S. Pat. Nos. 4,723,064 and 5,187,343. The tube-fill procedure is time consuming, uses excessively high amounts of the inert gas and does not always produce consistently good quality welds. U.S. Pat. No. 5,126,526 discloses a pair of sealing devices connected by a rod which enable an enclosure of a tube to be welded.
A more recent development employs inflatable bladders which are deployed on either side of a tube joint to be welded (i.e., see U.S. Pat. No. 5,100,043). Some of the inert gas used for shielding is also used to inflate the bladders and further inert gas is injected into the chamber created by the inflated bladders. Over time, the inert gas dilutes and later displaces the air in the weld area, thus creating an inert atmosphere for a subsequent weld operation. A serious drawback in this technique is that at least one bladder must be moved over hot metal during removal of the sealing structure, hence, damage results. A variation of the localized seal structure employs a collapsible cone (see U.S. Pat. No. 4,415,114) which is inserted into the tube and is opened (like an umbrella) to seal the area adjacent to the weld zone.
Double chamber arrangements have also been suggested to minimize the effect of leaks that may develop in the chamber immediately surrounding the weld region. See, for example, U.S. Pat. No. 5,231,258. In such a double chamber arrangement, inert shielding gas is present in both chambers so that leaks in the central weld chamber do not give rise to atmospheric contamination.
Diffusers have been suggested in the prior art for the purpose of reducing turbulence in flow of a shielding/purging gas. U.S. Pat. No. 4,916,281 describes the use of a diffuser for back purging and torch shielding in butt welding of a pipe. The '281 patent describes a pair of dams, with a diffuser attached to a bottom dam (for vertical welding). The upper dam is provided with a small hole to allow the gas to escape. In U.S. Pat. No. 5,235,152 a pair of ring seals within a pipe straddle a junction to be welded. A porous material is placed between the seals, and gas is supplied from inside the pipe and floods the seam to be welded (through the porous material). Because the ring seals of the '152 patent are positioned on either side of the weld region, removal of the structure causes at least one of the seals to ride over the weld area and to be possibly damaged as a result of the hot metal surface. U.S. Pat. Nos. 4,528,436 and 5,152,453 describe the use of diffusers to enable an even flow of an inert shielding gas during welding of tubes, pipes, rods etc. In both patents, the diffusers are positioned outside of the pipe, with the '453 patent indicating that the diffusers enable a laminar flow of inert gas along the pipe's external surface.
Weld shielding gases that are much denser and heavier than atmospheric gases are also known in the prior art and their advantages are discussed, for example, in U.S. Pat. No. 4,839,489. Use in welding applications of "neutral density" gases is also known (i.e. gas mixtures that have the same density as the surrounding atmosphere). U.S. Pat. No. 5,234,148 describes the use of such gas mixtures in shielding or purging applications, the proportion of the gas being controlled such that the gas mixture has a density approximately equal to that of the surrounding air. U.S. Pat. No. 5,234,148 also mentions the advantages of use of mixtures of gases having a density about the same as the density of atmospheric air. The '148 patent indicates that such gases may be used for open-butt welding where conventional vacuum or flow purging is not feasible. It notes that the sealing of a pipe is not necessary and that such mixtures enable welding in open-ended tubes. The '148 patent makes the point that since the density of the gas mixture is substantially the same as that of the ambient atmospheric air, the gas mixture remains at the welding zone so as to prevent air from affecting the weld quality.
The above noted prior art has a number of drawbacks. Some employ inflatable or fixed seals on either side of the weld joint which require, when the seals are withdrawn, that at least one of the seals is brought into contact with the welded region and any residual heat still present. Other systems require the use of slidable seals which are difficult to align and provide inconsistent results. Still others require that the entire tube be filled with an inert gas which is a wasteful process.
Accordingly, it is an object of this invention to provide an improved weld shielding apparatus and system particularly adapted to use in conduits.
It is a further object of this invention to provide an improved weld shielding method and apparatus wherein only a limited amount of inert shield gas is required.
It is still another object of this invention to provide an improved weld shield method wherein a significant reduction in purge time is achieved.